Sintautai plain landscape setup and development
The Suvalkija tectonic elevation with Sintautai landscape and Šešupė Plain had been formed by differentiated neotectonic movements. The orthogonal and diagonal tectonic faults (especially of SWNE direction) left a block step system in the crystalline basement, and they were active for several tectonic stages.
A general regularity in glacio-lacustrine plain is observed, i.e. prominent Upper Cretaceous remnant ridges are alternating with dozen metre deep inter-ridge depressions filled with till clay, glaciofluvial sand and gravel as well as glaciolacustrine clay and loam of small basins. A river valley element seems to be the most important from both points: orographic and due to differentiation of Quaternary formations. The ice flows ant rived valleys are genetically related. The ingressional tongues of plastic ice during the climatic phases played a significant role in formation of the plain.
A gentle westward sloping of the Šešupė Plain and smoothness of its surface seemed to be resulted just from an apparent glaciolacustrine levelling. Real reasons lurk in multi-layer sedimentation under ice lobes with a certain smoothing effect and mainly in sedimentation in sub-glacial lakes under the long-term ice cover. The ice tongue pulsation changed the environment and sediments: a small forward push of glacier (1), stagnation (2), dry cold climate period (3), ice melting and a rise of periglacial basin (4). During these phases, the sedimentation occurred most often under the long-lasting ice-shield.
Several kilometre wide clay areas in the plain are alternating with similar in size areas of sand and loamy sand of heterogeneous lithology formed by the meso-size tongues of ice related to outflow sections and later to river valleys.
The former models of deglaciation reported that the lower (80, 5060 and 40 m high) periglacial basins formed a gradually lowering cascade. Whereas the author of the present article in his model showed that the pulsation of glacial lobes complicated the lowering character of the cascade. On the other hand, the under-ice lakes used to reach the balance during the hydro-chemical optimum periods of high alkalinity rather than at the periods of hydro-dynamic calm. The significant role of the hydro-chemical factor is confirmed by the formation of calcareous concretions in clay at the lake bottom by coagulation. A hypothetical statement was made that the groundwater exchange intensified during the glacial periods, and the hydrocarbonate flows from shallow lying beds or ridges increased. The sub-glacial lakes were highly alkaline. The vertical circulation of water intensified due to hydrogeological windows typical in Šešupė Plain.
The Nopaitis meltwater valley inherited from earlier glacial periods is also of tectonic structure and a fact confirming valley inheritance.
Creation of drainage system in the Šešupė Plain helped to stop the gleying of clay soils, as it is an imperative in land management. Creation of Nova and Aukspirta valley nature reserves was a rather logical step. This is an example of a comprehensive hydrological interaction of rivers both with high and low yields.